The Gallery of Natural Phenomena

The earth, the sea, the sky - and beyond

Mercury and Venus appear to be the only planets in the Solar System without a moon to call their own. However, in the 18th Century, there were some doubts as to whether or not Venus had a companion. Several respected astronomers had reported observations of a mysterious object resembling a moon of the Planet of Love.*** On the morning of January 25, 1672, the famous astronomer Giovanni Domenico Cassini telescopically observed a body near Venus. “Venus was then horned, and this object, which was of diameter almost one quarter that of Venus, was of the same shape. It was distant from the southern horn of Venus a diameter of Venus on the western side.” Cassini watched this object for ten minutes until it was lost in the dawn. On the morning of August 28, 1686, using a 34-foot focal length telescope, Cassini saw the same or a similar object. It was an 'ill-defined light', which nevertheless 'seemed to imitate the phase of Venus'. Once again its diameter seemed about one-quarter that of Venus. Cassini observed for fifteen minutes, then left the telescope. Five minutes later he returned, but by then the dawn had become too bright.*** Just before sunrise on October 23, 1740, James Short, a well-known optician, while looking at Venus with a 16-inch focus telescope, saw 'a small star pretty nigh her'. He turned to another telescope, and, using a power of 240x, “to my great surprise found this star put on the same phasis (phase) with Venus. I tried another magnifying power of 140 times, and even then found the star under the same phasis. Its diameter seemed about a third, or somewhat less, of the diameter of Venus; its light was not so bright or vivid, but exceeding sharp and well defined. A line passing through the centre of Venus and it, made an angle with the equator of about 18 or 20 degrees.” Short observed the body for an hour until it was lost in daylight. He never saw it again.*** In December of 1757 it seemed that this minor astronomical mystery was about to be resolved. The Abbot Hell, of the Vienna Observatory, studying Venus with a 2-foot reflector, saw an ill-defined star like 'a little comet' near the planet. The 'comet' was not visible in other telescopes, so he decided it was a reflection from the inside of the telescope tube. Next year, in March, 'the illusion returned'. When the Abbot moved his eye toward the eyepiece, the appearance changed to “a perfect image of a satellite with the phase of the primary”. Experiments convinced him that the illusion was the result of reflection from his eye and the eyepiece lens.

*** As usual with mysteries, the matter was not quite so straightforward. Short had seen his object with two different telescopes, with various magnifications, and watched it for an hour. Abbot Hell's 'illusion' was only visible in one instrument, under limited conditions.*** On May 20, 1759 Meier, using a reflecting telescope of 30 inches focus, saw above Venus “a little globe of far inferior brightness, about 1½ diam. of Venus from herself.” Meier observed this for half an hour, and the position of the little globe with respect to Venus remained the same, though the position of the telescope had been changed. T.W. Webb pointed out that during such a long observation, “the eye must have been repeatedly removed and replaced, which could not have occurred without the detection of an optical illusion.”*** On May 3, 1761, Montaigne at Limoges saw a little crescent-shaped body about 22 minutes of arc away from Venus. As usual, it showed the same phase as the planet, and had one-quarter the diameter. He repeated the observation several times during the night, and on 4, 7, and 11 May (the intervening nights were cloudy) he saw the companion again, differently placed but still showing the same phase. On the 7th the object was seen distinctly even when Venus was not in the telescope field. The French Academie des Sciences was told that the satellite was one quarter the diameter of Venus, was as far from Venus as the Moon is from Earth, and had a period of 9 days 7 hours.*** A month later, on June 6, 1761, during the transit of Venus across the Sun, Scheuten, ‘an obscure German observer’ said that he observed a small black dot next to the black disc of Venus.

*** A satellite of Venus was observed by Roedkioer and others in Copehagen on March 3, 4, 9, 10, and 11, 1764. On March 4 the 'satellite' was one-quarter the diameter of Venus away from Venus on the left. It was visible in a second telescope, although not in two other instruments. Four different observers saw the object, one of whom was Horrebow, the Professor of Astronomy. They “satisfied themselves by several experiments...that it was not a deception.'*** On May 23, 1823, Venus showed an odd appearance to the Rev. T. W. Webb, the author of the famous astronomical guide, Celestial Objects for Common Telescopes. He observed, using a small telescope of just over one inch aperture, “a star…like Venus in miniature, preceding the planet…at a short distance…It was much smaller than Venus, perhaps one-third or one-quarter of its diameter”. The star ɛ Geminorum was near Venus at the time, but, at magnitude 3-4, it did not seem likely to Webb that the star could have been so conspicuous in the twilight.

***

Bianchini, the elder Herschel, Schroter (who observed Venus for 15 years), Harding, Struve, Lamont, Smyth, De Vico, Secchi and many other experienced observers never saw the satellite; but M. Houzeau (once Director of the Brussels Royal Observatory) was unwilling to believe that the observations of an object near Venus were 'all illusions', for all were made by celebrated astronomers or experienced observers. However, Houzeau said that the body could not be an actual satellite. The observed positions were impossible to reconcile with an orbit round Venus, and the mass of the planet “deduced from the least defective attempts would be 7 times the real amount.” He suggested that the object might be an intra-Mercurial planet, which sometimes would appear to be so near Venus as to be visible in the same telescopic field as that planet, and “it would appear beside the larger disk of Venus as a body of smaller size, presenting almost the same phase”. Such a planet could not appear further from the Sun than Mercury, and could only be seen near Venus when the intra-Mercurial planet was not very far from the Sun. If it was sometimes seen further from the Sun than Mercury, “it would be necessary to seek another hypothesis.” Houzeau compiled a table of the 7 best-known observations of the satellite, but “in every instance Venus was further from the Sun than an intra-Mercurial planet could possibly be.” His next hypothesis was that the body was a planet in an orbit just outside that of Venus, with a period of about 283 days. He named the object Neith, after “the mysterious goddess of Sais, whose veil no mortal raised.” The 7 observations seemed to support this idea. However, other observations, such as Roedkiaers' of March, 1764 make it impossible that two planets moving at such different speeds (5 revolutions of Venus would equal 4 revolutions of Neith) should have appeared to continue in conjunction for so long a time.

*** M. Paul Stroobant, in 1887, studied the Neith observations and concluded that most were of stars. Others were the result of telescopic defects. The only observations left unexplained were those made by Roedkioer at Copenhagen in March 1764. On March 4, 1764 Uranus was 10' apart from Venus. However, Stroobant and Meeus studied this conjunction and concluded that the object seen from Copenhagen 'could not possibly have been Uranus'. *** Something was seen. Not a star, not a known or unknown planet, not an asteroid, not an illusion, and not a satellite of Venus. What was it?